Cyclic AMP modulates the rate of 'constitutive' exocytosis of apical membrane proteins in Madin-Darby canine kidney cells.

Madin-Darby canine kidney and other epithelial cell lines (e.g. Caco-2, MCF-10A and MCF-7) develop intracellular vacuoles composed of apical membrane displaying microvilli (VACs) when impaired from forming normal cell-to-cell contacts. In a previous publication, we showed that VACs are rapidly exocytosed upon treatment with 8-Br-3',5'-cyclic adenosine monophosphate (8-Br-cAMP), a membrane-permeable analog of cAMP, and that this exocytosis correlates with variations in the cellular cAMP concentration in response to the cell-cell contacts. In the present work, we tested the hypothesis that cAMP may be a positive modulator of the 'constitutive' exocytic pathway. To mimic conditions in cells with incomplete intercellular contacts, the intracellular levels of cAMP were decreased by means of two independent approaches: (i) pores were induced in the plasma membrane with the polypeptidic antibiotic subtilin, thus allowing small molecules (including cAMP) to permeate and move out of the cytoplasm; and (ii) adenylate cyclase and protein kinase A were blocked with specific inhibitors. In all cases, the intracellular levels of cAMP were measured and, in porated cells, equilibrated to simulate the corresponding physiological intracellular concentrations. The decrease in cAMP within the physiological range resulted in a decreased rate of transport of an apical marker of the constitutive pathway (influenza virus hemagglutinin) from the trans-Golgi network to the apical plasma membrane. Likewise, the delivery of a number of cellular apical proteins to the plasma membrane was retarded at low cAMP concentrations. The inhibitors of adenylate cyclase failed to block basolateral delivery of vesicular stomatitis virus G protein. This differential modulatory effect may represent a differentiation-dependent control of the insertion of apical membrane in epithelial cells.